This invention relates to rectifying diodes suited for high speed power switching purposes.
Recently, the high speed switching technology has become popular in various fields dealing with power electronics. Typical examples of application are switching power sources, motor control and electronic control of various industrial machines.
The introduction of high speed switching into the field dealing with power meets the ends of power saving and size and weight reduction of apparatus which have long been desired. To these ends, however, great improvements in the characteristics of rectifying diodes are required.
The electric properties required for rectifying diodes for switching power sources are:
(1) low forward voltage (i.e., low loss), PA0 (2) short reverse recovery time (i.e., high speed), PA0 (3) high upper limit of permissible temperature (i.e., high operating temperature), PA0 (4) high breakdown voltage, and PA0 (5) low switching noise. PA0 for a silicon Schottky diode, EQU .rho.&gt;10 .OMEGA.cm EQU 0.1 eV&lt;.phi.h&lt;0.4 eV EQU 5 .mu.m&lt;W&lt;20 .mu.m; PA0 for a pn junction diode, EQU .rho.&gt;1 .OMEGA.cm EQU 0.15 eV&lt;.phi.h&lt;0.35 eV EQU 5 .mu.m&lt;W&lt;30 .mu.m.
It is not so easy to simultaneously meet all these requirements. Diodes are based on the most basic principles and structure among semiconductor elements, and unlike ICs it is difficult to vary their characteristics over wide ranges with corresponding variety of circuit constructions and patterns. Therefore, the gist of the improvement of power diodes is how to modify the basic operational principles and structure for obtaining the electric properties listed above to the utmost.
At present the diodes which are practically used for high speed power rectification are of two types, i.e., Schottky diode and pn junction diode. Each type has disadvantages as described below and its range of application is limited.
From the standpoint of high speed operation, the Schottky diode is presently most excellent since it is a majority carrier element having fast reverse recovering speeds. In addition, the Schottky diode has further advantages of lower forward voltage than that of the pn junction diode and capability of high efficiency operation. However, Schottky diodes conventionally used have a breakdown voltage which is about 50 V at the most, so that its use has essentially been limited only to low voltage applications. Also, the Schottky diode has a large junction capacitance and its upper limit of permissible temperature is low.
The disadvantages of the Schottky diode will now be explained in greater detail. The Schottky diode of a general structure comprises a low resistivity n-type substrate, an n-type epitaxial layer deposited on the substrate and a metal electrode or Schottky electrode forming a Schottky junction with the n-type epitaxial layer. The resistivity of the silicon n-type substrate is usually set to be 0.1 .OMEGA.cm or less preferably 0.02 .OMEGA.cm or less.
To provide for a high breakdown voltage with the above construction, both the resistivity and thickness of the n-type epitaxial layer have high values. Doing so, however, increases the resistance of the n-type epitaxial layer to greatly increase the parasitic voltage drop that develops across the n-type epitaxial layer when a forward current is passed. Therefore, there are restrictions on these parameters, and it has been common in the conventional power Schottky to set the resistivity and thickness of the silicon n-type epitaxial layer to 1 .OMEGA.cm or less and 3 .mu.m or less, respectively. For these reasons, the practical breakdown voltage has been within approximately 50 V as has been mentioned. In this respect, one may refer to "Theoretical Performance of the Schottky Barrier Power Rectifier" by D. J. Page, SSE Vol. 15, No. 5-C, page 509, and "Comparison of the pn Fast Switching Rectifier and the Schottky Rectifier" by Robert A Smith et al, IAS 76 Annual, page 61.
The inevitable reduction in thickness and resistivity of the n-type epitaxial layer in the conventional Schottky diode is due to the fact that because the Schottky diode is a majority carrier element and the conductivity modulation of the n-type epitaxial layer by means of minority carriers (hole) cannot be expected, the resistivity of the n-type epitaxial layer acting by itself as a diode series resistor must be reduced to decrease the forward voltage. As a result, breakdown voltage is decreased and junction capacitance is increased. A further technical consideration has been predominant in the past, according to which the presence of even a very slight quantity of minority carriers in the Schotky diode reduces the operating speed and has adverse effects (Refer to "Physics of Semiconductor Devices" by S. M. Sze, John Wiley & Sons, New York, 1969). For this reason, the injection of minority carriers has been avoided as much as possible by making the Schottky electrode have a potential barrier value of 0.4 eV or more against holes, and as far as the conventionally familiar technology is concerned, utilization of hole injection from the Schottky barrier to the n-type epitaxial layer has not been practiced.
In many fields dealing with power, pn junction diodes are often used because high voltages are involved. The pn junction diode has a small reverse recovery speed (about 0.5 .mu.S or more in terms of recovery time) due to the storage of minority carriers and is unsuitable for high speed operation.
A typical structure of the conventional pn junction diode comprises a low resistivity n-type substrate, a high resistivity p-type epitaxial layer deposted on the substrate and a p.sup.+ contact layer of high impurity concentration formed in the epitaxial layer. Further, metal electrodes are formed on surface of the n-type substrate and p.sup.+ contact layer, respectively. The p.sup.+ contact layer and the metal electrode thereon forms a so-called low-high electrode. The resistivity of the n-type substrate and p.sup.+ contact layer is usually set to 0.1 .OMEGA.cm or less, preferably 0.02 .OMEGA.cm or less. The resistivity of the p-type epitaxial layer is set to 1 .OMEGA.cm or more in order to ensure sufficient breakdown voltage. In this case, the impurity concentration in the p.sup.+ contact layer is set to 5.times.10.sup.18 cm.sup.-3 or more.
In the above prior art pn junction diode, minority carriers (electrons) injected from the n-type substrate during forward current conduction cannot jump over the low-high electrode and a great quantity of electrons is stored in the high resistivity p-type epitaxial layer, so that the reverse recovery speed is reduced. Further, a voltage drop across the low-high electrode is added.
The phenomenon described above is discussed in detail in "Minority Carrier Reflecting Properties of Semiconducto High-Low Junctions" by J. R. Hauser et al, Solid State Electronics, Vol. 18, p. 715 (1975).
For attainment of high speed operation, a gold doped diode has been manufactured wherein an epitaxial layer of a pn junction diode is doped with gold. In the gold doped diode, however, the upper limit of the permissible temperature is reduced, that is, one of the features of the pn junction diode is lost. Also, the forward voltage is increased and the uniformity of manufacture is reduced. Switching noise is also increased.